Digital printing systems can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art. The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format. More development is underway of production printers that require inkjet direct marking onto cut sheet media. This includes UV curable inks, solid inks and aqueous inks.
The output sheet can be printed on one side only, known as simplex, or on both sides of the sheet, known as duplex printing. In order to duplex print, the sheet is fed through a marking engine to print on the first side, then the sheet is inverted and fed through the marking engine a second time to print on the reverse side. These output sheets are transported to a stacker.
The purpose of the stacker is to compile printed sheets into a well-formed stack suitable to user end requirements, such as off-line finishing or bulk distribution. Current production printers are equipped with a high capacity stacker that produces a stack in which sheets can be optionally offset to one of two positions in the cross-process direction. It is desirable to have a stacker effective and reliable at speeds of at least 110 ppm or more.
A problem in a sheet stacking system utilizing a plurality of registration belts to align successive sheets onto the top of a stack is the development of high spots in the stack. High spots on a stack can progressively build up due to sheet curl, cockle, and edge damage. If a high spot on a stack develops, then the drive force imparted to successive sheets by the plurality of registration belts may become non-uniform, which results in sheets to be poorly aligned on top of the stack, resulting in an undesirable stack. Further, if the high spot builds to a sufficient level in which one or more registration belts completely lose contact with the top of the stack, then this results in an asymmetric loss of drive leading to skewed sheets and jams in the stacker.
Currently, to prevent this type of skew from occurring, the operator must have prior knowledge that a given print job will build up a stack with a high spot, and then manually limit the stack height, thereby keeping the high spots from building up beyond the latitude of the stacker.